In a large bowl, combine baking soda, cornstarch, citric acid, and Epsom salt.
If you are making bath bombs in different colors, divide your mixture evenly into one bowl per color. Add drops of colorant slowly until you reach your desired color.
Add essential oil (if desired) and melted coconut oil, dividing evenly among each bowl. Mix well.
Lightly spray mixture with witch hazel until it will hold its shape. Go sparingly with the witch hazel you won't need very much.
Mist pan with non-stick spray lightly. Press the mixture into the mold, using a spatula to press the mixture down as firmly as possible. Allow bath bombs to set fully before removing from mold (8-24 hours).
After removing from the mold, melt a few cubes of goat’s milk soap base in a bowl, according to the package instructions. Dip the top of each bath bomb into the melted soap and set aside a few minutes to allow to dry. It won’t take long. Then, re-dip each bath bomb in the melted soap and immediately add sprinkles. Allow the bath bombs to sit until they are fully set (about 15 minutes).
Notes
We used a lavender Epsom salt, that we found, but regular Epsom salt will work as well.